In recent years, in the field of a semiconductor laser device (LD; laser diode), a multi-wavelength laser device in which a plurality of light-emitting sections having different luminescence wavelengths are monolithically formed on the same substrate has been actively developed. The multi-wavelength laser device is used as a light source of an optical disc device, for example.
In such an optical disc device, a laser beam of a 790 nm band is used for the playback of a CD (Compact Disc) and also used for the recording and playback of a recordable optical disc such as a CD-R (CD Recordable), a CD-RW (CD Rewritable), or an MD (MiniDisc). Further, a laser beam of a 650 nm band is used for the recording and playback of a DVD (Digital Versatile Disc). By mounting the multi-wavelength laser device on the optical disc device, also with respect to any of plural types of existing optical discs, recording or playback becomes possible. By forming a laser device into a multi-wavelength structure in this manner, it becomes possible to further expand its use.
In a multi-wavelength laser device having a monolithic structure, in general, similarly to a single-wavelength laser device, a low-reflection film matched to the wavelengths of the respective laser beams is formed together on the entirety of a front end face of the laser device and a high-reflection film matched to the wavelengths of the respective laser beams is formed together on the entirety of a rear end face of the laser device. At this time, in order to obtain high reflectance, in general, the high-reflection film has a multilayer structure in which a low refractive index layer and a high refractive index layer are alternately laminated, and is constituted by the combination of materials in which a difference in refractive index between the low refractive index layer and the high refractive index layer becomes large (refer to JP-A-2008-16799 and JP-A-2010-171182).
Usually, in a multi-wavelength laser device which is used for the playback of an optical disc, in order to adjust alight output which is emitted from a front end face, emitted light from a rear end face is converted into an electric current by a photodiode and monitored. This monitor current depends largely on the reflectance of the rear end face. On the other hand, in a laser device, since a wavelength varies with a change in temperature, the larger the amount of variation in the reflectance of the rear end face with respect to a wavelength, the larger the change in monitor current with respect to the change in temperature of the device becomes.
Therefore, in the past, in order to make the amount of variation in the reflectance of a high-reflection film with respect to a change in wavelength as small as possible, as a material of the high-reflection film, a material in which a difference in refractive index between a low refractive index layer and a high refractive index layer becomes large has been used. For example, as a material of the low refractive index layer, Al2O3 (refractive index: 1.64) has been used, and as a material of the high refractive index layer, Si (refractive index: 3.3) has been used.